In polyurethane foam manufacturing surfactants are needed to stabilize the foam until the product-forming chemical reaction is sufficiently complete so that the foam supports itself and does not suffer objectionable collapse. High potency surfactants, generally understood to be those which give a high height of rise and little top collapse, are desirable because foams which collapse to a substantial degree before setting have high densities and objectionable density gradients.
However, surfactants are relatively expensive. Accordingly, foam manufacturers prefer to use as little as possible, employing only as much as required for desired performance. Manufacturers also desire that the properties of the final product be relatively constant as a function of the amount of surfactant used, so that surfactant levels can be varied in the manufacturing process to give good processing, without greatly affecting the properties of the product.
Silicone surfactants for polyurethane foam manufacture typically are materials having siloxane backbones and polyether pendant groups. They are of two types, nonhydrolyzable and hydrolyzable. The nonhydrolyzable surfactants, in which the polyether pendant groups are attached to the siloxane backbone by Si--C bonds, are generally believed to have high potency, but to produce "tight" foams with poor (low) breathability. Hydrolyzable surfactants, in which the polyether pendant groups are attached to the siloxane backbone by hydrolytically unstable Si--O bonds, are generally believed to have poor potency, but to offer good processing characteristics, and to produce foams with good (high) breathability.
Some investigators have questioned such generalizations and have attempted to develop silicone surfactants that have high potency, provide good processing characteristics, and produce foams having good breathability, by varying the structure of the surfactant appropriately. Examples of such materials are described in U.S. Pat. No. 4,147,847 which describes certain siloxane-oxyalkylene copolymer silicone surfactants having ratios of (CH.sub.3).sub.2 SiO.sub.2/2 units to (CH.sub.3)(R)SiO.sub.2/2 units in the range of3.5:1 to 15:1, for use in flame retardant polyurethane compositions. Another example is given in U.S. Pat. No. 5,145,879 which describes nonhydrolyzable silicone surfactants which have high potency, offer good processing characteristics, produce foams having good breathability, and which function well in flame retardant formulations. However, these formulations, unfortunately, require ancillary blowing agents, such as methylene chloride, to generate the desired foam density.
Most ancillary blowing agents have been found to contribute to the depletion of the ozone layer or air pollution and are being heavily regulated by legislatures. These ancillary blowing agents are becoming increasingly restricted due to concerns for ozone layer depletion, air pollution, worker exposure, and other health, safety and environmental issues.
The use of additional water (to react with the isocyanate to generate excess CO.sub.2) has been gaining popularity as the primary means to blow the foam. These systems are highly stressed and require surfactants that insure compatibility of the components with the higher levels of water as well as the desired foam performance features described above.
One type of surfactant that was found to work well in such situations is that described in U.S. Pat. No. 5,145,879. However, the teachings therein are limited to a narrow range of silicone architecture and are limited in molecular weights. This is believed to have been limited by the difficulty in handling the increased viscosity of higher molecular weight silicones. More importantly, the silicone surfactants described therein exhibited problems with foam performance, yielding coarse cells and/or center splits, despite being better than most surfactants at the high levels of water required in all-water blown foam systems.